With the generalization of mobile communication, digital signals modulated in a mobile terminal and a base station have to be amplified up to desired transmission outputs using a radio frequency (RF) power amplifier. In order to transmit a signal with no distortion, the RF power amplifier requires high linearity and high efficiency.
With the high efficiency of the power amplifier, research for an Envelope Elimination and Restoration (EER) based transmitter has been actively conducted. The EER based transmitter is a system for controlling the power supply of a power amplifier to follow an envelope of a transmitted signal. The EER based transmitter increases the amplitude of the power source of a power amplifier when the amplitude of a transmitted signal increases, and decreases the amplitude of the power source when the amplitude of the transmitted signal decreases. This method can enhance power efficiency compared to a method using a fixed power source that is designed considering the maximum amplitude of a transmitted signal. However, in order to prevent a distortion of an output signal of the power amplifier, an output signal of an envelope signal amplifier for controlling the amplitude of a power source of the power amplifier that should synchronize with a phase modulated signal is inputted to the power amplifier in the EER based transmitter. That is, the EER based transmitter needs timing adjustment between the output signal of the envelope signal amplifier and the phase modulated signal input to the power amplifier. A failure in accurate time alignment leads to a distortion of a spectrum characteristic of the output signal of the power amplifier.
A function of accurate time alignment between respective signals has to be realized to prevent the distortion of the spectrum characteristic of the output signal of the power amplifier. To realize the time alignment function, it requires to accurately measure a time taken to supply a baseband signal to a power amplifier via an envelope signal path and a time taken for a phase modulated signal to be up modulated and pass a power amplifier path, and requires to calculate a difference of time taken for transmission by path and adjust a delay by path.
For accurate time alignment, the conventional art has proposed several technologies for measuring a delay difference between an envelope signal path and a phase signal path and realizing time alignment on the basis of it. As for conventional technologies, there are a method for measuring a distorted signal generated due to a delay difference to measure the delay difference (US Patent Application Publication No. 2006/0246856 A1 entitled “TRANSMITTER APPARATUS”), a method for measuring a phase difference of a test signal (Korean Patent Application No. 10-2005-0003164 entitled “METHOD FOR TIMING ADJUSTMENT IN WIRELESS COMMUNICATION APPARATUS”), a method for delay measurement through correlation coefficient measurement (US Patent Application Publication No. 2006/0234652 A1 entitled “TRANSMISSION APPARATUS, COMMUNICATION APPARATUS AND MOBILE RADIO APPARATUS”) and the like. As for methods for correcting the thus measured delay, there are a method for performing time alignment by adjusting a phase of a digital clock (Korean Patent Application No. 10-2005-0003164 entitled “METHOD FOR TIMING ADJUSTMENT IN WIRELESS COMMUNICATION APPARATUS”), a method for controlling a delay using only a delay buffer for a digital signal path (US Patent Application Publication No. 2006/0234652 A1 entitled “TRANSMISSION APPARATUS, COMMUNICATION APPARATUS AND MOBILE RADIO APPARATUS”), a method for adjusting number of serial connection of power amplifiers for amplifying phase modulated signals (US Patent Application Publication No. 2006/0264186 A1 entitled “TRANSMITTER”) and the like.
However, it is difficult to derive a quantitative delay difference using the method for measuring a distorted signal generated due to a delay difference and performing time alignment and the method for measuring a phase difference of a test signal. The method for delay measurement through correlation coefficient measurement cannot set a delay of less than a clock period because a delay is set in a clock period unit.
Among the methods for correcting a delay, the method for adjusting a phase of a digital clock to perform time alignment on the basis of an estimated delay difference has a disadvantage of making it difficult to guarantee accurate operation performance depending on an operation characteristic of a data converter. The method for time alignment using a buffer cannot realize time alignment of less than a clock unit. The method for adjusting the number of serial connection of a power amplifier has a disadvantage of system size and cost increase, limited accuracy and the like.